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LMV324 Output Swing in Unity Gain

John Guy76
Prodigy 30 points
Other Parts Discussed in Thread: LMV324S, LMV324, LMV344

Hi

I am using a handful of LMV324s in a low frequency fixture. Supply voltage is +5V. Problem is on every instance of this on PCB, but we can discuss just one instance. U6-B, a unity gain buffer, sees a 0V (within a couple millivolts, best I can tell) to 900mV pulse, 20% duty cycle, 1/2Hz. Very pedestrian signal, right? The rise and fall time is about 1ms. The output swings from 42mV to 900mV, then back down to about 48mV, then a 300ms decay back to about 42mV.

I have verified the chip is indeed grounded. All other aspects of the amplifier seem valid, but it just does not seem to like to swing to ground. I have added 1k pull down resistors with no effect.

Your thoughts?LED Controller Board SCH -JRG May 25.pdf

  • 0
    TI__Mastermind 24985 points

    Hello John,

    Please refer to the output swing voltage for the device in the datasheet on page 6 of the datasheet.

    The output low level swing is typically 65mV. Therefore what you are measuring is probably about what is typical for the device.

    I am sorry for the delay in response, as we typically try to answer within 24 hours. 

    Please let me know if you have additional questions, and please continue to post on the forums if you need help, as we have product experts continuously monitoring these forums to help with any questions you may have.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Prodigy 30 points

    Hi Michael

    The key lies I believe in the circuit diagram for the device, showing an NPN low side device. So a pull down resistor must source all the bias current of the output stage due to base collector diode forward biasing near 0V. I guess that's why I got 40mV against they typical 65mV, I had 1k pull down resistors.

    I replaced the LMV324 with LMV344 with MOS output and the circuit works well.

    Thanks

    John

  • 0 in reply to John Guy76
    TI__Mastermind 24985 points
    John,

    I agree with you, and yes, the LMV344 has a much better output low voltage.
    I hope that device works for your application.

    Please let me know if you have additional questions.

    Best,
    Michael